The schematic of Prior Art FIG. 1 illustrates a 4-transistor emitter follower 100 including transistors Q1-Q4 and source and sink current devices I1 and I2. As will be appreciated, the emitter follower 100 provides many essential characteristics of a good output stage and therefore is commonly used as an output buffer stage in operational amplifiers. The emitter follower output current Ief is however limited by the product of the quiescent current of the current sources I1 and I2 and the current gain of the output transistor devices Q3 and Q4. This limitation can be particularly bothersome for high-speed complementary bipolar processes where beta PNP can be 20 or even lower, especially at cold temperatures.
The Prior Art teaches output buffer stages that are variations and improvements upon the emitter follower 100 of FIG. 1. For example, output current capability is often improved by adding another emitter follower stage, resulting in a "triple" buffer that provides higher output current at the expense of reduced voltage swing and increased quiescent current requirements. The reduced voltage swing is usually unacceptable for amplifiers that must operate from supplies of 5 volts or less.
The Prior Art also suggests that the output current capability of the 4-transistor buffer 100 may be improved by applying the output emitter follower collector currents to the inputs of current mirrors with outputs in parallel with the buffer output. This output stage can provide additional output current in direct proportion to the current mirror gain. However, additional quiescent current equal to the product of buffer output device quiescent current and current mirror gain is required.
The schematic of Prior Art FIG. 2 illustrates one class-AB output stage 150 that is a common variation upon the emitter follower 100 of FIG. 1. The output stage 150 includes a complementary 4-transistor emitter follower 100, common emitter transistor devices Q5 and Q6, and source and sink current devices 13 and 14. The class-AB output stage 150 provides improved output current capability but increases the required quiescent current and sacrifices output voltage swing capability.
The output stage 150 of FIG. 2 operates as follows. Current sources 13 and 14 source and sink, respectively, the quiescent current of the emitter follower devices Q3 and Q4 and provide the quiescent base current for common emitter output devices QS and Q6. All of the signal current in the collectors of the emitter follower devices Q3 and Q4 is available to drive the bases of the common emitter output devices Q5 and Q6. In essence, the common emitter output devices Q5 and Q6 are providing feed-forward current at the output.
In practice, the output stage 150 is non-functional. Temperature and device variations bring about intolerable swings in the quiescent current of common emitter output device Q5 and Q6. A means for controlling the quiescent current of the common emitter output devices that does not limit the signal current gain is required.
Prior Art FIG. 3 illustrates an output circuit 200 that provides control of the common emitter output devices in a manner that does not limit the signal current gain. Unfortunately, as will be described, the output circuit 200 has poor distortion characteristics. The output circuit 200 includes a class-AB output stage 150, parallel connected drivers Q1O and Q13, and a complementary pair of constant current transistors Q9 and Q14 that are operated as current mirrors together with transistors Q7, Q8, Q11, and Q12. The current mirror inputs are obtained from a relatively low current source/sink IREF. The transistor sizes are chosen so that the output pair quiescent current is the desired multiple of current mirror inputs. The constant current transistors Q9 and Q14 also source and sink the collector current of the emitter follower output devices Q3 and Q4, acting as equivalents of current sources 13 and 14 of FIG. 2.
Operation of the output circuit 200 may be more fully understood after reflecting upon the following. Consider a positive going signal applied to the input at VIN which results in sourcing output current to a load connected to VOUT. As load current begins to flow, the current through Q3 increases while current through Q4 decreases. The increasing current entering the collector of Q3 decreases the potential at the base of Q5 resulting in increased Q5 current which adds to the total current sourced to the load. The decreasing potential at the base of Q5 results in decreasing current in Q10. This reinforces the reduced current in Q4 resulting in lower potential at the base of common emitter output device Q6. The collector current in Q6 is reduced, further enhancing the net sourcing of current to the load. Note that the reduced potential at the base of Q6 also results in increased current in Q13, further reinforcing the net current sourcing action of the circuit.
Thus the output circuit 200 is capable of providing high output current drive, but only when the beta current multiplier of transistor Q5 is fully utilized. This means that Q3 must have a full current load. For the transistor Q3 to have a full current load, Q9 must be operating as a near perfect current source. For transistor Q9 to operate as a near perfect current source, transistor Q10 must transition from an ON to an OFF state. Any modulation of Q1O causes distortion in the output signal, but the ON/OFF transition perhaps the most. Therefore, the output circuit 200 provides high current gain for the cost of distortion.
Prior Art FIG. 4 schematically illustrates an output circuit 250 including a buffer output circuit 252 and a feed-forward control circuit 254. Reflecting upon the output circuits of FIGS. 1-3, those skilled in the art will recognize a pattern arising. The 4-transistor common emitter follower circuit 100 is utilized as a buffer output circuit and the improvement circuits added in FIGS. 2 and 3 are feed-forward control circuits intended to address the deficiencies of the buffer output circuit 100. Prior Art FIG. 4 illustrates one block diagram model for an output circuit having both buffering and feed-forward control. For example, in the output circuit 150 of FIG. 2, the transistor devices Q5 and Q6 feed-forward and make available additional current at the output Vout. The output circuit 200 of FIG. 3 provides a crude controller compensating for the lack of control over quiescent current within the output circuit 150.
What is needed is a proper biasing of the feed-forward circuitry that does not result in poor circuit distortion characteristics.